1. Field of the Invention
The invention relates to measurement of physical properties of matter. In particular, the invention relates to a system and method for measuring a glass transition temperature of a material such as a polymer or a composite incorporating a polymer.
2. Discussion of the Related Art
Advanced structures frequently incorporate polymers such as epoxy resins and exhibit a “glass transition temperature” commonly designated as “Tg”. The glass transition temperature of a material is useful for indicating its state, the degree of cure, effectiveness of curing agents, and softening point at elevated temperatures.
As a polymer is heated, it undergoes a transition similar to a thermodynamic second order transition. This involves a discontinuous change in a secondary thermodynamic quantity such as the expansion coefficient or heat capacity. The Tg is often referred to as the temperature region wherein a polymer goes from a glassy state to a rubbery state. Typically, the material will undergo a modulus change that can range over three orders of magnitude through this region. In resin systems, the Tg is influenced by, among other things, the composition of the resin molecule, the cross-link density, the polar nature of the resin molecule's functional groups, the curing agent or catalyst, and the cure time and temperature.
A material's glass transition temperature Tg therefore provides information about degree of cure of a resin and whether the formulation was properly prepared. As such, Tg is one of the most important quality control properties of a neat resin and/or composite. There are numerous examples in the development and failure analysis of composite hardware where problems attributable to chemistry, contamination, and processing were identified by the use of this test.
Known Tg measurement methods include those based on the material's heat capacity, its coefficient of thermal expansion, and its modulus of elasticity. These methods and some of their limitations are discussed below.
The Tg of a specimen can be tested using differential scanning calorimetry (“DSC”). In this process, changes in heat capacity can help identify the Tg as a function of temperature. However, in filled systems or composites, the reduced volume of the material that is typically analyzed will produce only a limited DSC signal that is often not very discrete.
The Tg can also be measured by following the coefficient of thermal expansion (“CTE”) for a resin using thermal mechanical analysis (“TMA”). A discontinuous change in CTE as a function of temperature usually indicates Tg. TMA characterizations are suitable for thicker unreinforced specimens. Notably, measuring CTE variations in high fiber volume composite materials with low CTE values is a much more complex and difficult task to perform.
Dynamic mechanical analysis (“DMA”) is one of the most commonly used techniques for Tg determination. DMA measures the response of a material to a sinusoidal or other periodic stress. Since the stress and strain are usually not in phase, two quantities can be determined: a modulus and a phase angle. Since the material usually undergoes a large drop in modulus through the Tg, the instrument can identify this point. This test is typically limited to specimens of a specific thickness and dimension (i.e. 1.0″×1.5″×0.25″). However, thin films are typically quite difficult to measure as are differences between localized areas.